Man

Man (hominids) Hominidae, the mammalian family that includes modern humans and their immediately extinct ancestors, contains four genera: Ardipithecus, Australopithecus, Paranthropus, and Homo. The family is distinguished from the African apes (their closest living relatives) by a number of anatomical and behavioural traits. The most important of these are: (1) possession of a relatively large brain in which the frontal and occipital lobes are especially enlarged, allowing more complex behaviour; (2) adoption of a bipedal gait, which involves a distinctive complex of skeletal features; and (3) a slow rate of postnatal growth and development, favouring complex social organization. The adoption of bipedalism as the main mode of locomotion was the fundamental anatomical and behavioural change that led to the divergence of the hominid lineage from that of the African great apes.

In recent decades, the volume of fossil material relating to the evolution of the hominids has greatly increased The modern multidisciplinary approach has begun to produce a clearer picture, not only of the hominids themselves, but also of the environments they inhabited. New and more precise dating techniques are making it possible to analyse how the hominids evolved anatomically and technologically against a background of palaeoenvironmental change. The intensive study of modern apes has established a whole set of links between anatomy and behaviour which can be used to interpret the fossil material. New biomolecular techniques have also added another avenue of research. While a number of areas of uncertainty remain, it is now apparent that the pattern of hominid evolution was more complex than had been thought (Fig. 1).

The most primitive hominid currently known, Ardipithecus ramidus, was found in a 4.4 million-year-old (Pliocene) horizon in Ethiopia. The pelvis and leg bones of A. ramidus are unknown, but cranial evidence hints that it was to some extent a biped. The key hominid locomotor adaptation, and consequently the divergence from the African ape lineage, therefore took place prior to 4.4 million years ago. This agrees with biomolecular data which supports a Late Miocene date (about 6 million years ago (Ma)) for the last common ancestor of the hominids and African apes. Ardipithecus ramidus may have been ancestral to all later hominids. New, but as yet undescribed, material should help clarify both relationships of this form with Australopithecus species and its mode of locomotion.

The ancestry of Ardipithecus is unknown and, without direct evidence, there has been considerable speculation on how and why the ancestral hominids became bipedal. The most popular hypothesis suggests that the shift in locomotor repertoire was promoted by an environmental change involving the break-up of the equatorial forest into discontinuous stands of trees. This might have forced the essentially arboreal proto-hominids to travel across open ground from one food patch to another. The available evidence indicates that Ardipithecus ramidus inhabited woodland environments. However, it is now beginning to appear more likely that bipedal locomotion evolved while the ancestral hominids inhabited wooded areas and not as an adaptation to open environments. Postural bipedalism, similar to that displayed by chimpanzees, could possibly have originated in arboreal or terrestrial feeding situations in wooded environments and was subsequently modified to locomotor bipedality. Whatever the cause, the adoption of bipedalism had profound behavioural implications. Bipedalism pre-adapted the early hominids to life in more open environments in a number of ways. These included freeing the hands for other activities, increasing the visual horizon for a ground-dwelling animal in flat country, providing an energetically efficient means of terrestrial travel between widely scattered food resources, and minimizing the area of the body exposed to solar radiation when the sun was overhead, thereby reducing the thermal stress experienced in open country.

The genus Australopithecus contains three species, all from African Pliocene sites: A. anamensis, known only from Kenya in beds between about 3.9 and 4.2 Ma old: A. afarensis, found in Ethiopia, Tanzania, and possibly Chad in horizons between 3.9 and 3.0 Ma old; and A. africanus, which inhabited South Africa between 3.2 and 2.3 Ma ago. All three forms are characterized by a chimpanzee-like skull with a large face attached to a relatively small cranium (Fig. 2). The cranial capacity (a measurement of the volume of the brain) was very similar to that of a modern chimpanzee. All three species were relatively slender and lightly built animals, standing between 1.0 and 1.5 m high. Australopithecus species exhibit a high degree of sexual dimorphism; among several morphological differences, the males are considerably larger than the females. Australopithecus postcranial skeletons demonstrate that these forms were upright bipeds. A trackway preserved in volcanic ash 3.2 Ma old at Laetoli in Tanzania confirms that australopithecines were bipedal striding animals with a foot similar to that of modern humans rather than that of the apes. However, they also had relatively long arms, and this, together with some details of the forelimb anatomy, indicates that they may have spent a substantial amount of time climbing in trees. Australopithecus species lived in a variety of habitats, ranging from forested areas to relatively open environments.

The three species of Paranthropus (P. aethiopicus, P. robustus, and P. boisei) are considered to represent a specialized group of hominids. Paranthropus aethiopicus and P. boisei are east African forms, while P. robustus occurs in southern Africa. They were small-bodied animals, ranging in height from 1.1 to 1.4 m, but their long bones were more robust than those of Australopithecus. They are distinguished by the presence of pronounced facial buttressing and cranial cresting, an extremely robust mandible, and massive cheek teeth (Fig. 2). These characteristics form a unique functional complex which was an adaptation to powerful or repetitious chewing. It is believed that these animals ate a wide range of plant foods, including nuts, seeds, and fibrous material. The postcranial skeleton appears to have been similar to that of Australopithecus and they also exhibit the same high degree of sexual dimorphism. Paranthropus diverged from the Australopithecus lineage prior to 2.6 Ma ago and survived until about 1.0 Ma ago.

Homo, the genus to which modern humans belong, appears in the east African fossil record about 2.4 Ma ago. Its origin was in some way related to a massive cooling of the world's climate which began about 2.6 Ma ago. The current model suggests that, as the east African climate became colder and more arid, the vegetation became sparser and this compelled the ancestral Homo to become more mobile and consume a more varied diet. The defining characteristics of the genus are in a state of flux but it appears to contain relatively large-bodied forms with a large cranial capacity relative to the body size, thick cranial roof bones, and a parabolic dental arcade. The increase in cranial capacity was the second key event in hominid evolution, providing our ancestors with greater behavioural, and technological innovation. Some authorities recognize as many as seven species: Homo rudolfensis, H. habilis, H. ergaster, H. erectus, H. neanderthalensis, and H. sapiens.

The oldest species, Homo rudolfensis, occurs in eastern and southern Africa in horizons dated at between 2.4 and 1.8 Ma. It exhibits some traits resembling Australopithecus, including the skull shape (Fig. 2), but the cranial capacity is relatively large and the brain had a fully Homo-like pattern. Like H. rudolfensis, H. habilis occurs in eastern and southern Africa but at sites dating between 1.9 and 1.5 Ma old. The cranial shape (Fig. 2) in this species resembles that of later Homo species but the cranial capacity is smaller than that of H. rudolfensis. The postcranial skeleton of both species is poorly known but appears to have been rather Australopithecus-like.

The earliest stone implements found so far in eastern Africa are about 2.5 Ma old and it is usually assumed that the appearances of Homo and artefacts are linked. It is probable that the appearance of lithic artefacts was preceded by a long period in which plants were used as raw materials for tools, similar to the digging sticks and twig probes employed by chimpanzees today. The earliest tool kit, termed the Oldowan, is characterized by choppers, crude implements fashioned from cobbles by striking off flakes from either side of the stone to create a sharp edge. Analysis of archaeological sites, which are essentially associations of artefacts and mammalian bones, supports the view that, while meat had begun to be used as a regular source of food, the hominids were scavenging carnivore kills.

About 1.9 Ma ago, the third species in the genus, Homo erectus, evolved, probably in eastern Africa. This had a long, low-vaulted and thick-walled skull with a massive, straight brow ridge (Fig. 2). The brain was larger and the face less projecting than in the earlier forms. This species was similar in stature to modern humans, and sexual dimorphism was much less than that exhibited by Australopithecus and Paranthropus species. Some authorities suggest that the earliest material is sufficiently different from later specimens to warrant allocation in a separate species, H. ergaster, but this conclusion is not universally accepted. The appearance of H. erectus has been linked to tectonic uplift and consequent climatic shifts in eastern Africa, but the evidence is not entirely convincing. Shortly after its appearance, this species migrated out of Africa, probably before 1.8 Ma ago; its remains are found in Israel, Georgia, China, Vietnam, and Java. While fossils of this species do not occur in Europe, archaeological finds suggest that it periodically inhabited the Mediterranean region. Its appearance outside Africa marks the third key event in hominid evolution. Surprisingly, in spite of its wide geographical distribution, H. erectus remained little changed for over a million years. Associated with some, but not all, H. erectus sites is a new, larger, and more sophisticated tool kit, the Acheulian, the characteristic implement of which is the large hand axe. Archaeological data indicate that this species was a hunter–gatherer. Some late H. erectus groups were able to control fire and possibly cooked some of their food.

In the late Early Pleistocene, a form intermediate between H. erectus and H. sapiens, sometimes called archaic H. sapiens or H. heidelbergensis, is found in Africa, Asia, and Europe. Regional differences can be detected amongst the available fossil sample and it is probable that a number of separate species may eventually be recognized. The details of the transition from H. erectus remain unclear and its date appears to have varied from region to region, ranging from about 800 000 to about 300 000 years ago. All the transitional forms retain the low forehead of H. erectus, but the thick brow ridges curve over the eye sockets, the rear of the skull is more rounded, and the cranial capacity is larger (Fig. 2). Some populations of this group adapted to cool temperate conditions and they were clearly able to cope with and exploit a great diversity of habitats. The archaeological materials found in association with this species also exhibit variation, the tool kit being modified to suit local requirements. The first evidence for the construction of shelters appears at this time.

In Europe and western Asia about 300 000 years ago, Homo heidelbergensis evolved into a distinctive form, H. neanderthalensis. This species was heavier and more muscular than modern man, but, contrary to popular opinion, it had a completely upright stance and a physiology close to our own. Cranially, its main distinguishing feature was a long skull with a flat top, resembling H. erectus (Fig. 2). The cranial capacity was, on average, actually larger than that of modern man. The distinctive anatomy of the Neanderthals appears to be an adaptation to the glacial climate experienced intermittently in its geographic range during the Late Pleistocene. Associated with the Neanderthals is the Mousterian tool kit, comprising smaller and more refined implements than the Acheulian. The Neanderthals buried their dead, sometimes with grave goods, implying that they had some form of ritual or spiritual aspect to their life.

The place, time, and cause of the emergence of Homo sapiens, the fourth and final critical step in hominid evolution, remains controversial. On balance the available evidence supports the hypothesis that our species evolved over 100 000 years ago, either in southern or eastern Africa, and subsequently migrated to other parts of the world. It had reached parts of the Middle East by 90 000 years ago, China by 67 000 years ago, and Australia by 40 000 years ago. It did not enter Europe until after 35 000 years ago, after which it either displaced or replaced Neanderthals. Possibly it did not possess the requisite technology to deal with the climatic conditions in this area until this late date. Early H. sapiens is associated with a sophisticated stone and bone technology, exhibiting a large number of local variants.

Allan N. Insole

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